Answer
See answer below.
Work Step by Step
a) $Sn^{2+}$ is oxidized because tin's oxidation number goes from +2 to +4, in $Sn^{4+}$, thus it's the reducing agent.
$CrO_7^{2-}$ is reduced because chromium's oxidation number goes from +6 to +3, in $Cr^{3+}$, thus it's the oxidizing agent.
b) $FeS$ is oxidized because iron's oxidation number goes from +2 to +3, in $Fe^{3+}$, and sulfur's oxidation number goes from -2 to +6, in $SO_4^{2-}$, thus it's the reducing agent.
$NO_3^{-}$ is reduced because nitrogen's oxidation number goes from +5 to +2, in $NO$, thus it's the oxidizing agent.
